Simplify effect change checking (and make effect cycle detection more accurate as a side-effect)

.changeset/pretty-students-look.md

@@ -0,0 +1,5 @@
+---
+"@preact/signals-core": patch
+---
+
+Simplify effect change checking (and make effect cycle detection more accurate as a side-effect)

packages/core/src/index.ts

@@ -66,11 +66,7 @@ function endBatch() {
 			effect._nextBatchedEffect = undefined;
 			effect._flags &= ~NOTIFIED;
 
-			if (
-				!(effect._flags & DISPOSED) &&
-				effect._flags & OUTDATED &&
-				needsToRecompute(effect)
-			) {
+			if (!(effect._flags & DISPOSED) && needsToRecompute(effect)) {
 				try {
 					effect._callback();
 				} catch (err) {
@@ -619,7 +615,7 @@ function Effect(this: Effect, compute: () => void) {
 	this._cleanup = undefined;
 	this._sources = undefined;
 	this._nextBatchedEffect = undefined;
-	this._flags = OUTDATED | TRACKING;
+	this._flags = TRACKING;
 }
 
 Effect.prototype._callback = function () {
@@ -643,15 +639,14 @@ Effect.prototype._start = function () {
 	prepareSources(this);
 
 	/*@__INLINE__**/ startBatch();
-	this._flags &= ~OUTDATED;
 	const prevContext = evalContext;
 	evalContext = this;
 	return endEffect.bind(this, prevContext);
 };
 
 Effect.prototype._notify = function () {
 	if (!(this._flags & NOTIFIED)) {
-		this._flags |= NOTIFIED | OUTDATED;
+		this._flags |= NOTIFIED;
 		this._nextBatchedEffect = batchedEffect;
 		batchedEffect = this;
 	}

packages/core/test/signal.test.tsx

@@ -524,6 +524,28 @@ describe("effect()", () => {
 		expect(fn).to.throw(/Cycle detected/);
 	});
 
+	it("should throw on indirect cycles", () => {
+		const a = signal(0);
+		let i = 0;
+
+		const c = computed(() => {
+			a.value;
+			a.value = NaN;
+			return NaN;
+		});
+
+		const fn = () =>
+			effect(() => {
+				// Prevent test suite from spinning if limit is not hit
+				if (i++ > 200) {
+					throw new Error("test failed");
+				}
+				c.value;
+			});
+
+		expect(fn).to.throw(/Cycle detected/);
+	});
+
 	it("should allow disposing the effect multiple times", () => {
 		const dispose = effect(() => undefined);
 		dispose();